By means of freezing microtomes of the aforementioned type thin section specimens are produced from tissue samples, which are subsequently observed under a microscope for diagnostic purposes. For this, the tissue samples are firstly frozen in a cooling device, also referred to as cryostat, and then cut by means of a cutting device. The cooling device and the cutting device are arranged together in a working chamber of the freezing microtome.
The cooling device usually comprises freezing ribs cooled by means of Peltier elements, to which slides holding the tissue samples are attached. This type of sample cooling has the disadvantage that during the freezing process elongated ice crystals are formed in the tissue due to the crystallization of the cellular and extracellular water, which perforate the cells and united cell structures of the tissue and affect the histomorphology adversely. Such tissue damages are also referred to as freezing artifacts. A possible adverse affect by freezing artifacts consists e.g. of the damaged cells releasing endogenous enzymes, which become active during an RNA preparation and thus degrade the substrate used in the preparation. This reduces the sample quality and as a consequence makes a reliable diagnosis more difficult.
To prevent freezing artifacts, it is alternatively suggested to freeze the tissue samples externally, i.e. outside of a microtome, directly by means of liquid nitrogen or by using nitrogen-cooled isopentane. However, the use of liquid nitrogen is complex and expensive. Further, freezing of the tissue samples outside of the microtome involves the risk to mix up the tissue samples.
Regarding prior art, it is further referred to U.S. Pat. No. 6,615,592 B2. Therein a cooling device is described, wherein a liquid coolant is led over the tissue samples to freeze them.